This invention relates to scanning radio receivers and, more particularly, to scanning radio receivers particularly useful on the frequencies assigned to the Public Safety Radio Services.
Scanning radio receivers are well known in the prior art and have found particular utility for the reception of radio signals on the frequencies assigned by the United States Federal Communications Commission to the Public Safety Radio Services. In the past, many such radio receivers used crystals as the tuning element to provide the necessary local oscillator signals and required the presence of one crystal for each frequency the receiver was capable of tuning. Examples of such receivers are those shown in U.S. Pat. Nos. 3,531,724 to G. H. Fathauer, 3,665,318 to S. J. Hoffman, et al., 3,714,585 to R. C. Koch, 3,725,788 to G. H. Fathauer, 3,794,925 to K. Imazeki, 3,801,914 to K. Imazeki, 3,821,651 to G. H. Fathauer, et al., 3,873,924 to G. H. Fathauer, 3,883,808 to J. E. Boone, 3,824,475 to P. W. Pflasterer, and 3,987,400 to G. H. Fathauer. Recently, scanning radio receivers using frequency synthesizing techniques have been provided which eliminated the need for a large number of crystals in radio receivers capable of being tuned to a large number of frequencies. Such receivers are shown in U. S. Pat. Nos. 3,937,972 to S. C. Snell, 3,961,261 to P. W. Pflasterer, 4,000,468 to J. R. Brown, et al., 4,027,251 to G. H. Fathauer, et al., 4,114,103 to P. W. Pflasterer, and 4,123,715 to G. H. Fathauer. There has also been provided by the prior art scanning radio receivers using frequency synthesizing techniques wherein the frequency synthesizing circuitry was controlled by the operation of a processing means such as a microprocessor. Exemplary radio receivers of this last mentioned type are disclosed in U. S. Pat. Nos. 3,962,644 and 4,092,594 both to W. Baker as well as the presently pending U.S. patent application Ser. Nos. 847,497 of G. H. Fathauer, et al., 847,566 of G. H. Fathauer, 905 of W. L. Williamson, et al., and 1,013 of A. Khan, et al.
Scanning radio receivers utilizing frequency synthesizer circuitry operated under the control of a microprocessor have proven to be extremely useful and advantageous in large part due to the great degree of flexibility of operation given by the use of a microprocessor controller. One mode of receiver operation which was not available as a practical matter prior to the introduction of such receivers was the "search" mode wherein the user may cause the receiver to successively and automatically tune to adjacent ones of the Public Safety Radio Service frequencies until it becomes tuned to a frequency at which a signal is received.
One of the troublesome aspects of such receivers which has been evident in the past is the unintentional generation of spurious signals which may cause unintended and undesired results in the operation of the receiver. Specifically, such spurious signals may cause the receiver to respond as though it were receiving a signal at the frequency to which it is tuned when no such signal is in fact being received; in this instance, the spurious signals are commonly referred to as "birdies". Birdies are a particular problem when a scanning radio receiver is used in a search mode of operation because the presence of a birdie will cause the receiver to stop its successive and automatic tuning from frequency to frequency with the result that the receiver "hangs up" on the frequency at which the birdie exists. Generally once this occurs, the receiver may be made to resume its search operation only by intervention of the listener to manually cause the receiver to be tuned to another frequency.
In the design of prior scanning radio receivers some degree of attention was given to the reduction of the number of birdies present but there was little or no recognition of the specific nature of items which caused the birdies or of possible ways to reduce the birdies. It was generally known that the physical placement of the electrical components on the receiver's electric circuit board, the length and placement of electrical conductors, etc., may have an effect on the presence and strength of birdies but the particular causes escaped attention.
One such cause relates to the requirement of lowering the frequency of relatively high frequency local oscillator signals down to a frequency which could be handled by economically practicl frequency synthesizer circuitry. To be somewhat more specific, one previous design arrangement for public safety service scanning radio receivers used a first intermediate frequency of approximately 10.8 MHz and a single voltage controlled oscillator (VCO) to generate the first local oscillator signal required for reception of signals in the public service radio H, U, and T bands of approximately 150-174 MHz, 450-470 MHz, and 470-512 MHz, respectively. In order to generate the first intermediate signals on the H band, the VCO output signal was applied directly to a mixer, while on the U and T bands the VCO output signal was first tripled in frequency before application to a mixer. Thus, the VCO output signal needed to be variable in frequency over the range of approximately 135-167 MHz to tune the entire frequency range of the H, U and T bands. However, that VCO frequency range was above the practical limit of frequencies which could be handled by economically practical digital frequency synthesizer circuitry. One method used to reduce the VCO output signal to a more workable frequency was to mix its output with a signal at approximately 133 MHz so that the difference frequency output signal was in the range of approximately 2-34 MHz, a much more easily handled frequency range. However, this mixing down of the VCO signal was known to be a source of many spurious signals and birdies. The requirement of the 133 MHz oscillator signal and mixer also introduced needless and undesirable circuit complexity and expense.
Various expedients have been used in the past for dealing with the problem of birdies. Tables listing a receiver birdie frequency have on occasion been provided to the receiver users so that they could be careful to select the frequency ranges to be searched so as to exclude birdie frequencies. In the application Ser. Nos. 000,905 and 1,013 referred to above, a separate memory facility was provided whereby the user could "lockout" any birdie frequency he encountered during a search of a frequency band to disable the receiver from stopping the search operation on subsequent searches through that same band. While these expedients dealt with the birdie problem with some degree of success, they did nothing toward reduction of the birdies themselves.
Further, scanning radio receivers of the microprocessor controlled frequency synthesizer circuitry while having many operational advantages associated therewith have the corresponding disadvantage of a high degree of circuit complexity. Unless positive measures are taken, this high level of complexity may result in an undesirably elaborate circuit design which would be unnecessarily and perhaps prohibitively expensive because of the large number of parts required, the labor expense of assembling such a large number of parts, and the problems of circuit and component reliability which result from the use of a large number of parts. One method known for reducing the complexity of an electrical circuit from the standpoint of the number of separate electrical components which must be assembled into a complete unit is to manufacture at least portions of the circuit in integrated circuit form. However, when this approach is taken with a scanning radio receiver, the design of the receiver itself and the selection of those portions thereof to be put in integrated circuit form must be carefully coordinated always keeping in mind the constraints imposed by the various integrated circuit manufacturing techniques which are economically available.